1. Technical Field
The present invention relates generally to plain bearings and to their method of manufacture and more particularly to multilayer engine bearings of the type having a metal backed layer of copper-lead or aluminum alloy lining overplated with a softer bearing metal for use in internal combustion engines.
2. Related Prior Art
Plain engine journal bearings for use in high load engine applications, such as connecting rod, main and accessory or balance crankshaft bearings of high torque diesel or high performance gasoline engines, typically include a base lining member having a lining of either copper-lead or aluminum alloy formed on a surface of a steel backing. The lining is overplated with a softer bearing metal typically comprising a single layer of a lead-tin-copper alloy having a thickness of about 25 .mu.m. Often, a nickel diffusion barrier or copper bonding layer is interposed between the lining and overplate. As a final step, the bearing is typically coated with a micro-thin layer of tin or lead-tin flash plating having a thickness of about 1 .mu.m or less. The flash plating is primarily cosmetic, giving the product a bright, pleasing appearance. It also provides a level of corrosion protection to the steel backing. The micro-thin flash plating applied to the overplate quickly dissipates within the first few hours of engine break-in, and as such does not contribute to the bearing properties of the overplate.
In service, such multilayer crankshaft bearings are subjected to dynamic loads that vary in magnitude and direction due to the inertial loads applied by the piston and connecting rod mechanism and by the cylinder gas. The softer overplate layer enables the bearing surface to continually change and conform under high load forces to any misalignments or changes in profile or loading of the member being journaled, so that the loads are distributed across a greater surface area of the bearing. This property is generally known as "conformability". The overplate also allows any foreign particles of dirt or metal that may come between the bearing surface and the member it journals to become embedded or absorbed into the bearing surface, so as to protect the bearing and journal from excessive wear or damage. This property is known as "embedibility".
It is generally accepted that conformability and embedibility are dependant upon overplate thickess, with a thicker overplate being preferred. It is also generally known that as the thickness of the overplate increases, so does the susceptibility to bearing fatigue (i.e., the fracturing of the bearing surface when under load). Resistance to fatigue cracking requires that the bearing surface exhibit sufficient tensile strength to enable it to undergo minor configuration changes without cracking. Thus, it is necessary to balance the competing properties of conformability/embedibility and fatigue resistance when designing an engine bearing, particularly one that is to be subjected to high dynamic loading.
For many high load engine applications, it has been found that the single layer, 25 .mu.m thick lead-tin-copper overplate described above provides excellent conformability and embedibility characteristics and good fatigue resistance. However, as the output and efficiency of engines continually increases, the dynamic loads placed on the crankshaft bearings also increase, as does the potential for bearing fatigue. It has been found that under extreme loading conditions the conventional bearings described above having a single layer overplate of lead-tin-copper are prone to fatigue. Efforts to alleviate fatigue by simply decreasing the thickness of the lead-tin-copper overplate layer to less than the 25 .mu.m thickness while at the same time preserving an acceptable level of conformability and embedibility have been largely unsuccessful to date.
Thus, there exists a need in the industry for an improved journal bearing that can perform under such extreme high dynamic loading conditions without experiencing fatigue and while maintaining good conformability and embedibility characteristics.